The purpose of this project is to understand the regulation of the synthesis of milk proteins and their mRNAs in rat mammary gland at different stages of gestation and lactation. The studies include: (a) measurement of the content of alpha-lactalbumin and caseins - 42K, 29K and 25K - along with the levels and activities of their mRNAs in the mammary gland at various stages of gestation and lactation; (b) the rate of synthesis of alpha-LA and caseins in the explant cultures of these glands; (c) identification of the precursors of alpha-LA and casein mRNAs and (d) the influence of hormones, in particular prolactin, on the synthesis of these proteins and their mRNAs in the glands at different stages of gestation.